Biomedical subjects

Alexandre Malta Rossi

Publications and source records attributed to Alexandre Malta Rossi.

4 recordsLinked to original sources

PubMed2006

A structural analysis of lead hydroxyvanadinite.

Hydroxyvanadinite, Pb(10)(VO(4))(6)(OH)(2), was prepared by the co-precipitation method and analyzed by X-ray absorption spectroscopy (XANES, EXAFS), infrared spectroscopy, Raman scattering and X-ray diffraction (XRD). The results showed that the structure is very similar to that of vanadinite, Pb(10)(VO(4))(6)Cl(2), with space group P6(3)/m (176) and cell parameters a = 10.2242(3) A and c = 7.4537(2) A. A Rietveld refinement of the structure was performed using vanadinite as the starting model and fixing the geometry of the vanadate ion as a rigid body. First-principles Density Functional embedded cluster models are developed to analyze electronic structures, bonding, and densities of states. Interaction of Pb with the OH channel anion is examined in detail, as an important structural feature. A periodic band structure approach was used to obtain a further estimate of relaxed atomic coordinates.

Algorithms↗

PubMed2003

Dissolution properties of calcium phosphate granules with different compositions in simulated body fluid.

Calcium phosphate granules were produced through the calcination of three hydroxyapatites (HAs) at 1150 degrees C: nearly-stoichiometric (NS-HA), calcium-deficient (CD-HA), and carbonated (Carb-HA). The characterization using scanning electron microscope (SEM), X-ray diffraction (XRD), and Fourier-transformed infrared spectroscopy (FT-IR) showed that those granules exhibit strong differences in chemical composition, surface texture, and dissolution behavior. Sample dissolution in a simulated body fluid (SBF) and precipitation of a calcium phosphate layer on the granule surface were followed up for 7 days by chemical analysis of calcium content in SBF. After 80-min reaction, precipitation of new calcium phosphate phases predominates over the dissolution of original granules. Comparison between SEM images and XRD/FT-IR analysis revealed that the composition of the calcium phosphate layer and its precipitation rate depend on the HA composition and its heat treatment. Calcined carbonated apatite induces the precipitation of an octacalcium phosphate (OPC) layer, whereas a carbonated apatite layer is formed from calcined-deficient HA. The calcined nearly-stoichiometric HA exhibits low efficiency in producing carbonated apatite layer.

Biocompatible Materials↗

PubMed2002

Studies on the mechanisms of lead immobilization by hydroxyapatite.

The sorption of lead by synthetic hydroxyapatite (HA) from solutions containing Pb2+ initial concentrations up to 1770 mg L(-1) was studied. X-ray diffractometry (XRD) associated with Rietveld methodology for refining the spectra pattern was used in order to characterize the mechanisms of lead uptake. It is shown that the dissolution of hydroxyapatite is followed by the formation of a solid solution, Pb(10-x)Ca(x)(PO4)6(OH)2, with Pb ions mostly occupying Ca(II) sites. The Ca/Pb molar ratio of this solid solution decreases continuously until it reaches the structure of a pure hydroxypyromorphite. The cell parameters and the crystallite mean size behavior of both mineral phases reinforce the hypothesis that hydroxypyromorphite, PbHA, formation is the end of a process in which Pb(10-x)Ca(x)(PO4)6(OH)2 crystallites are continuously dissolved and recrystallized producing crystals with lower calcium content. Combination of Inductively Coupled Plasma spectrometry (ICP), chemical analysis, and XRD results permitted the conclusion that lead ions are not completely immobilized by precipitating Pb(10-x)Ca(x)(PO4)6(OH)2. Additional surface mechanisms also contribute to Pb2+ uptake. During Pb2+ sorption process, pH variations of the solution phase showed a more complex pattern than previously reported. Contribution of surface mechanisms, in addition to the hydroxyapatite dissolution, could explain this behavior.

Biocompatible Materials↗

PubMed2002

Hydroxyapatite deposition by electrophoresis on titanium sheets with different surface finishing.

Hydroxyapatite coatings are commonly applied to metallic biomedical implants to accelerate osseointegration. These coatings, usually produced by plasma spray techniques, can be obtained by alternative processes, like biomimetic process, electrolytic deposition, or electrophoretic process as well. Electrophoretic deposition of hydroxyapatite exhibits several advantages like simplicity and low cost. In this article, titanium sheets with three different surface finishing were coated with hydroxyapatite by using electrophoresis. Surface treatments include: (1) abrading with SiC paper; (2) abrading with SiC paper plus electrolytic etch with H3PO4 solution; and (3) blasting with alumina powder followed by etch with a solution containing H2O2 and HF. Stoichiometric hydroxyapatite was used to coat titanium sheets. Blasted samples were also coated using a calcium-deficient hydroxyapatite. SEM, XRD, and FTIR were employed to characterize titanium substrates and coatings produced. Results show that electrophoretic process can produce a uniform thin layer, satisfactorily adhered, of hydroxyapatite on treated titanium samples. Furthermore, sintering at 800 degrees C do not promote the decomposition of calcium-deficient hydroxyapatite.

Biocompatible Materials↗